Evolution: The Whale.

whaleResurrecting The Mammoth >

Darwin matters because evolution matters. Evolution matters because science matters. Science matters because it is the preeminent story of our age, an epic saga about who we are, where we came from, and where we are going.

-Michael Shermer

The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of land mammals. Their terrestrial origins are indicated by:

  1. Their need to breathe air from the surface.

  2. The bones of their fins, which resemble the limbs of land mammals.

  3. The vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish.

The question of how a group of land mammals became adapted to aquatic life was a mystery until discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.

The molecular data is supported by the recent discovery of Pakicetus, the earliest proto-whale (see below). The skeletons of Pakicetus show that whales did not derive directly from mesonychids. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids. Proto-whales retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today’s artiodactyls and perissodactyls became herbivores later in their evolution. By contrast, whales retained their carnivorous diet, because prey was more available and they needed higher caloric content in order to live as marine endotherms. Mesonychids also became specialized carnivores, but this was likely a disadvantage because large prey was not yet common. This may be why they were out-competed by better-adapted animals like the creodonts and later Carnivora which filled the gaps left by the dinosaurs.

1024px-Cetaceans.svg

“Cetaceans” by I, Chris huh. Licensed under CC BY-SA 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Cetaceans.svg#/media/File:Cetaceans.svg

Cetacea-evolution Cladogram_of_Cetacea_within_Artiodactyla  Evolutionary_pathway Mystice_pelvis_(whale)

Whales, like all mammals, evolved from reptiles, amphibians, and fish. Thus, over hundreds of millions they left the sea, grew legs, grew fur, and evolved lungs. Then they returned to the sea, lost their legs and fur, but kept their lungs.

A simply staggeringly complete fossil line..

More amazingly, Dr. Hans Thewissen, perhaps the leading specialist in whale evolution, tells us the transition from a completely land animal to a completely marine animal took only 8 million years.

On top of this, the end of whale evolution—the cetaceans, the order to which whales belong—are often more adept at life in the sea than fish! (Otters can outswim many fish, too!)

Cetacea

Cetaceans are mammals, the only mammals that live (all the time) in the water. The evolution of mammals will be covered elsewhere, so let’s skip that whole portion of the evolution of whales.

There are areas, from over 50 million years ago, where scientists are disagreed (and very excited about continuing to learn more) over which order whales evolved from. However, once we hit the pakicetids, 53 million years ago, the evidence starts nailing down the series for us.

Since we are excited to be able to go back 6 to 7 million years with man, the 53-million-year story of whale evolution is remarkably complete!

Summation of Whale Evolution

This series of fossils takes us from a hoofed land animal to a marine animal with back legs that cannot support it in just a few million years. The spine has changed, the limbs have shortened, the nostrils have moved, the skull has changed in shape, and the brain has changed in its function.

Along the way, all these species are linked by a special set of ear bones unique to this lineage. The timing of the fossils is right, and the progression is evident. It would be hard to imagine asking for better evidence of whale evolution.

This series of fossils is a powerful argument for evolution.

Nothing can be imagined more useless to the animal than rudiments of hind legs entirely buried beneath the skin of a whale, so that one is inclined to suspect that these structures must admit of some other interpretation. Yet, approaching the inquiry with the most skeptical determination, one cannot help being convinced, as the dissection goes on, that these rudiments [in the Right Whale] really are femur and tibia. The synovial capsule representing the knee-joint was too evident to be overlooked. An acetabular cartilage, synovial cavity, and head of femur, together represent the hip-joint. Attached to this femur is an apparatus of constant and strong ligaments, permitting and restraining movements in certain directions; and muscles are present, some passing to the femur from distant parts, some proceeding immediately from the pelvic bone to the femur, by which movements of the thigh-bone are performed; and these ligaments and muscles present abundant instances of exact and interesting adaptation. But the movements of the femur are extremely limited, and in two of these whales the hip-joint as firmly anchylosed, in one of them on one side, in the other on both sides, without trace of disease, showing that these movements may be dispensed with. The function point of view fails to account for the presence of a femur in addition to processes from the pelvic bone. Altogether, these hind legs in this whale present for contemplation a most interesting instance of those significant parts in an animal — rudimentary structures. – John Struthers, 1899

Part 2 To follow:

Resurrecting The Mammoth >

Advertisements

One response to “Evolution: The Whale.

  1. Pingback: Resurrecting the Woolly Mammoth: The Genetic Age | TheZenith·

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s